Not applicable.
Not applicable.
The present invention relates to cutting tools for milling internal or external threads in metallic workpieces or workpieces constructed of other materials. The present invention more particularly relates to thread milling tools having one or more helical flutes on a cutting portion of the tools. The present invention may be incorporated on thread milling tools for milling, for example, straight or tapered threads.
Thread cutting tools are known for cutting either internal threads in predrilled holes in a workpiece or external threads on a perimeter of a workpiece. These thread milling tools are typically known as xe2x80x9cthread millsxe2x80x9d. A typical thread mill has a shank portion and a thread cutting portion. Thread mills generally produce higher quality threaded workpieces when compared to other threading tools, such as taps.
The thread cutting portion of a thread mill may include thread-milling teeth and flutes. A typical thread mill may have between two to four flutes but may include more than four flutes for certain sizes and milling applications. Flutes may be provided on thread mills to provide cutting edges and to channel the chips produced during the milling operation out of a hole that is being threaded. The flutes defined in thread mills may be straight or oriented in a spiral or helical pattern having either a right-handed or a left-handed direction. Straight flutes lie generally parallel to the longitudinal axis of the thread mill extending from the end of the cutting portion and along the cutting portion. Helical or spiral flutes may be defined by a helix angle and wrap around the external surface of the thread mill. As used herein, the helix angle is the constant angle between and the central axis of the cutting tool or any line parallel to the central axis. Those of ordinary skill will understand and may readily determine the helix angle for a particular cutting tool.
Helical flutes defined in the cutting portion of a thread mill may produce a lifting action on the chips produced during the threading operation. The lifting action tends to force the chips through the flute and out of the hole that is being threaded. Helical flutes may also be advantageous, for example, when threading holes having keyways or other interruptions. The helical arrangement of the flute causes the cutting teeth to meet the interruption progressively, thus cutting more smoothly and being less subject to shock.
The thread-cutting teeth of thread mills may be configured to form threads defined by ISO or NPT standards or may have any other thread shape. The thread-cutting teeth have a configuration complementary to that of the threads to be formed on the inner surface of the bore or the external surface of the workpiece. The rows of cutting teeth are separated by the flutes defined in the cutting portion of the thread mill.
The shank portion of the thread mill may be mounted on, for example, the spindle of a milling machine, machining center, or computer numerically controlled milling machine. Thread mills are generally used on machines allowing motion along three axes, which allows the thread mill to move in the helical path required of the thread mill in operation. The thread mill rotates about its longitudinal axis, rotates around the center of the perimeter of the workpiece to be threaded, and also moves in an axial direction relative to the workpiece.
Multi-fluted thread mills work similarly to threading taps, but their design and operation differ in many respects. A tap operates primarily like a reamer that may be plunged very quickly into the workpiece to form threads. The thread form profile on the outside diameter of the tap is ground in with a spiral pattern that equals the pitch of the desired resultant thread pattern on the workpiece. Thus, the tap is literally pulled forward as it is twisted into the workpiece and, therefore, the feed rotational direction usually must be reversed in order to remove the tap from a blind threaded bore. Typically, two or three taps must be used to produce a complete thread. Thread mills, however, are milling tools, and they remove material chip-by-chip as the threads are milled in the workpiece. Thread mills usually have smaller diameters than a bore in a workpiece to be threaded. Although the thread profile is also ground on the outside diameter of thread mills, similar to taps, the threads are in line with each other and are not helically oriented. Therefore, a thread mill must be helically interpolated into the workpiece to form the desired resultant helical threads. Thread mills have cutting edges on each thread-cutting tooth are formed with a rake angle on the cutting portion of the thread milling tool.
Examples of known thread milling tools include the combined hole making and threading tools described in U.S. Pat. Nos. 5,413,438 and 4,761,844. A combined hole making and thread milling tool is provided with an elongated straight body having a shank at one end, and a hole drilling portion and a thread making portion along a working portion of the tool. The thread making portion of the tool has a maximum diameter equal to the maximum diameter of the hole drilling means so that the thread making portion can enter the hole without interference while the hole is being drilled. The thread making portion of the tool may have straight or helically disposed flutes. The bore is formed by the hole drilling portion and is then threaded using the thread making portion by combination of simultaneous rotary, axial, and orbital motion of the tool. The orbital motion of the tool is a movement of the axis of the tool about the center line of the hole, namely, along a circle whose center lies on a center line of the hole. The combined rotary, axial, and orbital motions of the combined tool cuts an internal thread on the inner surface of the hole.
U.S. Pat. No. 5,733,078 provides a drilling and thread milling tool wherein the cutting portion of the tool includes thread-milling teeth on an outer circumferential surface and also includes one or more end-cutting edges on a distal face of the tool. Each end-cutting edge has a maximum diameter which is intermediate in length to a minor diameter and a major diameter of the thread-milling teeth on the same tool. This drilling and thread-milling tool allows for the formation of a threaded hole close to an edge of the workpiece by reducing the minimum required wall thickness between the major diameter of the thread being generated and the edge of the workpiece.
In order for thread milling tools to operate efficiently, the chips generated during threading must be efficiently removed when a hole in a workpiece is being threaded. Chip entrapment or inefficient chip removal is a major cause of increased power consumption, reduced quality of the milled threads and tool wear. Thread mills used for threading blind bores are typically designed with flutes so that the chips are lifted out of the hole or, if enough clearance can be provided at the bottom of the hole, the chips are pushed ahead of the cutting tool. When the flute design of the cutting tool is such that the chips are lifted out of the hole by the lifting action of the flutes, it may also be helpful to direct a jet of cutting fluid into the bore through an internal passage within the tool. This flow of fluid both cools the workpiece and cutting tool and also assists in flushing the chips from the bore through the flutes. By improving upon the flute design of conventional thread mills, increases in threading efficiency and thread quality may be achieved.
Accordingly, it would be advantageous to provide an improved design for a thread mill that will more efficiently convey chips generated during threading operation. In that way, the efficiency of the threading operation will be improved, and the quality of the resulting threads also will be enhanced.
The present invention provides an improved thread milling tool for generating internal and/or external threads in a workpiece. The thread milling tool of the present invention includes a proximal shank portion and a distal cutting portion. The cutting portion of the thread milling tool comprises at least one thread milling tooth. At least one helically oriented flute is defined in the cutting portion. The helix angle of the flute defined in the cutting portion is greater than or equal to 20xc2x0.
A thread milling tool according to the present invention may also include a tapered region defined on the cutting portion wherein a diameter of the cutting portion tapers along an axial direction on at least a portion of the thread milling tool. The proximal shank portion may be attached to a device, such as, for example, a milling machine, a machining center, or a computer numerically controlled lathe with milling capabilities, that may translate the tool along a helically interpolated path appropriate to form the desired threads. The cutting portion may form internal or external threads in the workpiece as the cutting portion moves relative to the workpiece.
The thread milling tool of the present invention provides enhanced milling performance during thread formation relative to prior art helically fluted thread mills, which have flutes with smaller helix angles. The thread milling tool of the present invention forms more accurate thread parameters and thread profiles, resulting in a smoother threading process including smoother chips, and produces a generally smoother surface on threaded holes. The enhanced performance of the thread milling tool of the present invention may reduce the cutting forces necessary and vibrations created in the thread milling operation and, therefore, may enhance tool life and the tendency of chipping of the thread milling tool during threading.